1. Technical Field
This invention relates to digital radiography apparatus for processing a plurality of X-ray images to produce a clear diagnostic image.
2. Description of the Prior Art
In the X-ray diagnostic field, a recent development is an apparatus which develops a diagnostic image of a patient (or other object) by illuminating the patient with X-rays and then using a television camera employing an X-ray image intensifier to detect the X-ray transmission image of the patient. The most widely accepted of these apparatus employs the so-called digital subtraction process, in which the final diagnostic image displayed is the difference between X-ray images of a selected region of the patient obtained before and after the injection of a radiopaque, or X-ray contrast, medium into the region.
The subtraction process involves first producing an X-ray image of the region before the injection of the contrast medium. This first image is used as a "mask image" to be subtracted from other images. After the injection of the X-ray contrast medium, a second X-ray image of the same region, the contrast image, is produced. Both the mask image and the contrast image are digitized; and then they are subjected to digital subtraction. For example, in angiography of the cranium, digital subtraction is carried between an ordinary cranial X-ray image made before the injection of the radiopaque medium and an X-ray contrast image made after injection in which, in effect, the cranial X-ray image and an angiographic image are superimposed. In this way, the data concerning the cranial X-ray image is eliminated.
One difficulty with this subtraction process is the short period during which the contrast medium is effective. Conventionally, therefore, in obtaining a diagnostic image of a region of a patient which moves very slowly (for example, blood vessels), the contrast medium is injected at a time t.sub.0 as shown in FIG. 1; and, after the lapse of several seconds in which the contrast medium is allowed to spread, X-ray images are taken at intervals of, for example, 1 second and used to develop mask image M and sucessive contrast images S.sub.1, S.sub.2, S.sub.3, . . . In order to avoid exposing the patient to a large dosage of radiation, the X-rays are generated as pulses by using a tetrode between the high voltage generator and the X-ray tube to switch the power supply to the tube. Because tetrodes are expensive and suffer from the typical short service life of all vacuum tubes, this arrangement is undesirable.
Furthermore, an additional problem is presented when one attempts to obtain a diagnostic image of a region which moves relatively rapidly, such as the heart. In this case, as shown in FIG. 2, mask image M is obtained before injection; and contrast images S.sub.1, S.sub.2, S.sub.3, . . . are obtained afterward at intervals as short as 33 ms. However, when X-ray images are to be obtained at such short intervals, a tetrode is incapable of rapidly switching the power supply and still providing stable X-ray tube drive pulses, for reasons involving the well known operating characteristics of the tetrode. Consequently, as shown in waveform a of FIG. 2, the patient is continuously irradiated with X-rays throughout the period during which image processing is performed; and detection of separate contrast images is achieved by electrical sampling within the image processing unit itself. Because of the large amount of radiation to which the patient is thus exposed, there is an urgent requirement for development of a technique whereby stable X-ray tube drive pulses are produced with a small pulse repetition interval (PRI) as shown in waveform b of FIG. 2.